1. Field of the Invention
The present invention relates to a device and a method for distinguishing clearly, separately and automatically by a remote control without interrupting service, faults, whether the faults exist in an optical fiber cable or in another optical apparatus, such as an optical receiver employed in an optical transmission system.
2. Prior Art
Until now, the method for detecting whether a fault occurred on the optical fiber cable or on the optical receiver in the optical transmission system, that is, the method for distinguishing a fault between the optical fiber cable and the optical receiver is provided in Japanese Patent Application No. 58-71089.
FIG. 1 is a block diagram showing the optical transmission system and the fault distinguishing device used for carrying out the above mentioned conventional method. As shown in FIG. 1, the optical transmission system comprises an optical transmitter 1 for transmitting optical signals, an optical fiber cable 2 used as transmission lines and an optical receiver 3 for receiving optical signals. An optical filter 4a is located adjacent to the optical transmitter 1 in the optical fiber cable 2. On the other hand, an optical filter 4b, which has the same construction as the optical filter 4a, is located adjacent to the optical receiver 3 in the optical fiber cable 2. As illustrated in detail in FIG. 2, the optical filter 4b is constructed such that it passes a test light .lambda..sub.t for detecting the fault positions on the transmission lines but blocks and reflects a communication light .lambda..sub.o of the data transmission. Reflector 5 is disposed right behind the optical filter 4b and reflects almost perfectly test light .lambda..sub.t incident thereon through the optical filter 4b. A reference numeral 6 denotes a light pulse testing apparatus. This light pulse testing apparatus 6 is composed of a light source 7 emitting test pulse light .lambda..sub.t an optical multiplexer/demultiplexer 8 for joining and branching lights and a light pulse receiver 9 receiving test light .lambda..sub.t reflected by the reflector 5 and transmitted back through the optical fiber cable 2.
According to the above mentioned conventional method, in the time of detecting a fault position, if a fault, such as a break of the optical fiber and the like happens on the optical fiber cable 2, it can be confirmed that the cause of the fault exists on the optical fiber cable 2, since the reflected test light .lambda..sub.t is not received by the light pulse receiver 9.
On the other hand, when the reflected test light .lambda..sub.t is received by the light pulse receiver 9, it can be confirmed that the cause of the fault exists in the optical receiver 3, and not in the optical fiber cable 2.
However, with the above described conventional method, the respective connections of the optical filter 4b with the optical fiber cable 2, with the optical receiver 3, and that with the reflector 5 are such that insertion losses are 3 dB or greater, owing to the constructions of the lens optical systems. Similarly, as the optical filter 4a and the light pulse testing apparatus 6 is connected with the lens optical system, the insertion losses therein are 3 dB or greater.
Thus, miniaturization as well as reduction of manufacturing costs for transmission system has been difficult to achieve.
In addition, the devices manufactured by such conventional method have generally demonstrated significant optical losses.
Also, with the above described conventional method, although it can detect that the cause of a fault is in the optical fiber cable, it cannot detect the position of the fault.
Further, the above conventional method is unsuitable for applying to a branching optical transmission paths available for picture distribution service.